The present invention relates generally to a method and apparatus for adjustably supporting a pipeline relative to a ramp of a lay vessel. More particularly, the invention relates to a method and apparatus which can be employed to vary the vertical elevation of a pipeline relative to the ramp of a lay vessel to maintain a suitable curvature in the pipeline as it passes from the ramp to an associated stinger.
In recent years a great deal of commercial acitivity has begun focusing on the oceans. In particular, as supplies of petroleum indigenous to the major industrial countries have diminished and the countries having significant surplus reserves have become more nationalistic, exploration of the geological strata underlying the oceans has intensified. Such exploration is continuing and at present is being pressed into ever deeper areas of the oceans.
Oil fields discovered beneath the oceans must be serviced to remove the oil produced. Commonly, pipelines are laid on the ocean floor and are used to convey the oil from place to place. These pipelines are often put in place by means of specialized vessels referred to as lay vessels. The pipelines are often enormous both dimensionally and in weight and stretch over greatly varying terrain and depths.
At least three different types of lay vessels are currently in use. One type is commonly referred to as a barge hull. This type of vessel is large and shallow in draft. A second type is referred to as a ship hull. These vessels may be ships which have been converted for the express purpose of laying pipe or may be of entirely new construction. The third type of vessel in common use is referred to as a seim-submersible. This vessel entails elongated, horizontal pontoons and vertical stabilizing columns and is quite stable in heavy seas.
While the three different types of lay vessels differ considerably in overall configuration, several structural and operational characterisitics are common to each. All commonly assemble the pipeline on board from relatively short segments of pipe supplied by a smaller servicing vessel. All entail a lengthy ramp along which the pipeline moves longitudinally as it is being made up and put in place on the floor of the body of water. All three types of vessels normally employ a stinger trailing from the stern to assist in supporting and reducing the free-standing depth, i.e., the unsupported length, of the pipeline as it passes from the ship to the floor of the body of water. By reducing the free-standing depth of the pipeline, there is less tendency for the pipeline to buckle.
Various ramps, each differing principally in configuration, may be employed in each of the three different types of vessels. The most common types of ramps are those which are horizontal, slightly inclined relative to the main deck of the vessel, or gently curving in a concave, downward manner. These different types of ramp commonly extend along the side of the lay vessel. Due to the number and degree of separation of stations frequently required in constructing a pipeline, e.g., six to eight stations located at roughly 40-foot intervals, a ramp is normally quite long.
Stingers commonly employed to support a pipeline extend from the end of the ramp adjacent the stern of the lay vessel downwardly into the water. The joint connecting the stinger to the vessel affords articulation in at least a vertical plane. The length and inclination of the stinger relative to the vessel and ramp is dependent upon the structural characteristics of the pipe and various environmental factors including the depth of the water.
It is of great importance that the pipeline be properly supported at each of the various stations along the ramp. For instance, it can be appreciated that each successive length of pipe added to the pipeline must be accurately aligned with the preceding segment. Additionally, it may be useful to X-ray the welds joining individual segments of pipe to check for structural soundness. This operation may require accurate positioning of the pipeline. These functions may be facilitated by support shoes disposed periodically along the ramp.
Perhaps most importantly the curvature of the pipeline as it passes from the ramp to the stinger must be carefully controlled to avoid buckling. The overall configuration of this curve along the ramp and stinger of the lay vessel is commonly a function of the inclination of the stinger and the depth of the water in which the pipeline is being laid. The curvature necessary to avoid buckling of the pipeline as it passes onto the stinger can be maintained in part by raising and lowering the pipeline relative to the ramp to vary the degree and extent of the curve. The support shoes mentioned earlier are thus also useful in controlling the curvature of the pipeline.
Each of the support shoes is commonly independent of the others. Individually, the support shoes vary the elevation of various points of the pipeline above the ramp and thereby collectively regulate the curvature of the pipeline. By independently varying the elevation at which each support shoe supports the pipeline, the curvature thereof can be shifted to some extent along the ramp of the lay vessel and the sharpness of the transition from the ramp to the stinger can be controlled.
The individual shoes commonly in use are installed in the ramp at various points and each generally entail some sort of supportive trough or cradle which fits under the pipeline above the ramp of the lay vessel. The cradle of each support shoe is generally supported by a stanchion assembly consisting of a hydraulic cylinder guided by a telescopic sleeve. Support shoes of this and several other types have for various reasons not been entirely successful and have presented a number of serious problems.
A major problem encountered in connection with support shoes of the prior art is that the installation of the supportive stanchions associated with these devices has necessitated the formation of an opening in the ramp. The body of the stanchion is generally inserted in the opening and then anchored. This is perhaps most often true in the case of a support shoe in which the supportive stanchion is formed by a hydraulic cylinder and guide sleeve assembly. The disposition of the supportive stanchion in an opening in the ramp presents a difficult problem with regard to access to the stanchion. Workmen may be hampered in servicing, repairing, or replacing a support shoe of this type. The existence of an opening in the ramp may also allow a vessel to take on water or at least will form cavities tending to catch and retain water. This phenomenon would be a particular problem in areas such as the North Sea where weather can be quite rough.
The pipelines which are laid by a lay vessel are often enormous not only in size but in weight as well. Thus, loads of considerable magnitude may be applied to a cradle of a support shoe and ultimately to the supportive stanchions associated with the cradle. The matter is further complicated by the fact that a pipeline is often placed in tension to allow a decrease in the length of the stinger in use and a corresponding increase in the free-standing height of the pipeline. The enormous weight and loading applied in tensioning the pipeline thus may impose large normal loads on the cradle or trough supporting the pipeline. Thus, considerable amounts of drag or resistance to the longitudinal movement of the pipeline along the cradles may be developed.
The drag or resistance developed upon movement of the pipeline may be exerted in an essentially orthogonal manner relative to a supportive stanchion and may thus development significant bending moments therein. Bending moments of this character when combined with the large loading resulting from the weight or tensioning of the pipeline may precipitate the failure of a supportive stanchion. The failure of a support shoe in this manner might well result in the buckling or fracture of the pipeline.
A further problem which may be a corollary of that just discussed resides in the lateral instability of the support shoes, that is, the tendency of the support shoes to deflect in directions transverse to the longitudinal axis of the ramp. Support shoes employing hydraulic cylinder and guide sleeve assemblies as the supportive stanchions may be particularly subject to this problem. A lay vessel may be subjected to considerable buffeting in heavy seas particularly in inhospitable areas such as the North Sea. Considering this buffeting and the great weight of the pipeline, and the tensioning, thereof, it will be appreciated that the lateral instability of the support shoes may present a very serious problem. Not only does lateral instability render the stanchions vulnerable to buckling, but the accurate alignment or positioning of the pipeline at various stations along the ramp may be disrupted.
Many support shoes of the prior art afford a simple one-to-one ratio between the extension or contraction ot the telescopic stanchions and the resulting movement of the pipeline. A pipeline may require substantial variations in the elevation thereof above the ramp as the inclination of the associated stinger varies. Likewise, operations of the various stations along the ramp may require substantial alteration in the elevation of the pipeline. The problem is best phrased perhaps in terms of a deficiency in the degree to which the supportive stanchion can be extended or contracted to move the pipeline. In other words, in order to effect a sufficient change in the elevation of the pipeline an inordinately long telescopic stanchion may be required. Excessive length combined with high axial loads contributes greatly to lateral instability and the vulnerability of the assembly to buckling, particularly if the telescopic stanchion is comprised of a hydraulic cylinder and guide sleeve assembly. The length of a piston and cylinder assembly also presents problems in connection with the installation thereof in the ramp.
Many support shoes of the prior art may tend to develop excessive or unnecessary concentrations of stress in the ramp or the associated structural framing. This is particularly a problem in connection with support shoes entailing vertically telescopic stanchions. In these devices only a limited area may be available for contacting the ramp or associated structural framing and as a result undue stress concentrations may be developed. These stress concentrations may generally be of a cyclic character and are therefore potentially quite damaging.
Each vessel and each ramp of the various types has different structural characteristics which must be accommodated in installing support shoes. This is true whether the support shoes are installed during construction of the vessel or in replacing shoes on an older vessel. Because of factors relating to shape and mode of operation, many support shoes of the prior art of necessity penetrate the surface of the ramp or otherwise require special adaptations of the normal structure of the ramp. Each support shoe of the prior art thus may not constitute a complete unit in itself. Thus, the support shoe is not modular in the sense that it could be secured directly to the surface of the ramp without the need of any special structural adaptation of the lay vessel. Furthermore, support shoes of the prior art which cannot be assembled directly on the surface of the ramp may not be sufficiently flexible to render them amenable to use in the full spectrum of vessels and ramps already in use. Lay vessels are extremely costly and the difficulties suggested in the preceding exacerbate this problem.
In most cases the enormous loading which may be exerted on a support shoe is received by a cradle and transmitted into a supportive stanchion through a simple pivot pin. In such arrangements there may exist insufficient bearing area for the pivot pin to transmit the loading without introducing undue wear or strain to the end of a stanchion. Furthermore, a joint employing a simple pivot pin may not afford proper centering of the cradle relative to the ramp and remaining portions of the support shoe. A joint of this type may also be difficult to repair or replace.
The problems suggested in the preceding, while not exhaustive, are among many which tend to reduce the effectiveness and desirability of support shoes of the prior art. Other noteworthy problems may also exist; however, those presented in the discussion above should be sufficient to demonstrate that support shoes appearing in the prior art have not been entirely satisfactory, particularly with regard to the various problems just discussed.